Reaction products of silicic acid esters and organic tin compounds

ABSTRACT

Hydroxyl terminated diorganopolysiloxanes may be crosslinked with a mixture containing (1) a product obtained from the reaction of (a) silicic acid esters with (b) an organic tin compound of the formula 
     
         R.sub.2 Sn(OCOR.sup.1).sub.2                               (I), 
    
     where R represents a butyl or an octyl radical and R 1  represents a monovalent hydrocarbon radical having from 1 to 15 carbon atoms, in which at most one of the valences of the carbon atom that is bonded to the carboxyl group is saturated by a carbon atom other than that of the carboxyl group, and (2) an organic tin compound of the formula 
     
         R.sub.2 Sn(OCOR.sup.2).sub.2                               (II), 
    
     where R is the same as above and R 2  represents a monovalent aliphatic hydrocarbon radical having from 3 to 15 carbon atoms, in which at least two of the valences of the carbon atom that is bonded to the carboxyl group are saturated by two carbon atoms other than that of the carboxyl group.

The present invention relates to silicon-tin compounds and moreparticularly to silicon-tin compounds that are obtained from thereaction of silicic acid esters and organic tin compounds and their useas crosslinking agents for organopolysiloxane compositions.

BACKGROUND OF THE INVENTION

Compositions obtained from the reaction of silicic acid esters such astetraethyl orthosilicate, and organic tin compounds such as dibutyltindilaurate and their use as crosslinking agents for organopolysiloxanecompositions to form elastomers are described in German Patent No.1,167,527 to Farbenfabriken Bayer Aktiengesellschaft. U.S. Pat. No.3,186,963 to Lewis also discloses a catalyst system for vulcanizingsilicone elastomer stocks at room temperature in which the catalyst isprepared by heating for at least 15 minutes at a temperature of from 80°to 200° C. a tin salt of a carboxylic acid and an alkylsilicate orpartial hydrolysis products of said silicates.

U.S. Pat. No. 3,927,052 to Vizurraga discloses a polymerization catalystfor preparing polyesters which is obtained from the reaction of a firstsilicon compound of the formula ##STR1## with a second compound of theformula ##STR2## in a mole ratio of from 1.1 to 20 moles of the secondcompound per mole of the first compound for from two to six hours, inwhich M may represent a tin atom, R₂ and R₄ are chlorine or hydroxygroups and the other R substituents are selected from the groupconsisting of alkyl, acyl, alkoxy, aryloxy and hydrogen, provided thatat least one of R₅, R₆, R₇ and R₈ is not hydrogen.

Another reference which discloses silicon-tin compounds as condensationcatalysts in the preparation of organopolysiloxane elastomers in U.S.Pat. No. 4,137,249 to Wohlfarth et al. in which the silicon-tincompounds are prepared by reacting a silane of the formula R_(a)Si(OR⁴)_(4-a) with a dialkyltin salt of the formula

    R.sup.1 R.sup.2 Sn(OOCR.sup.3).sub.2

wherein R represents an alkyl or phenyl radical, R¹, R² and R³ are alkylradicals, R⁴ is a monovalent hydrocarbon radical, a is 0 or 1 in a molarratio of (4-a) moles of dialkyltin salt per mole of silane at atemperature of from 50° to 160° C., while removing from 80 to 100percent of the theoretical amount of by-product ester formed from thereaction.

The products obtained from the reaction of silicic acid esters and anorganic tin compound of this invention have certain advantages over thesilicon-tin compounds described in the prior art in crosslinkingdiorganopolysiloxane-based compositions to form elastomers. For example,diorganopolysiloxane based compositions containing the product obtainedfrom the reaction of a silicic acid ester and an organic tin compoundcan be crosslinked much more rapidly without decreasing the pot-life ofthe composition. Moreover, the crosslinking agents obtained from thereaction of silicic acid esters and an organic tin compound may bestored at room temperature or heated to a temperature up to 60° C.without a substantial decrease in the crosslinking rate or withoutsubstantially altering the pot-life.

Therefore, it is one of the objects of this invention to provide acrosslinking agent for organopolysiloxane compositions. Another objectof this invention is to provide a crosslinking agent for roomtemperature vulcanizable diorganopolysiloxane-based compositions. Stillanother object of this invention is to provide a crosslinking agentwhich is obtained from the reaction of a silicic acid ester and anorganic tin compound. A further object of this invention is to provide acrosslinking agent which may be heated up to about 60° C. withoutdecreasing the rate of crosslinking or the pot-life.

SUMMARY OF THE INVENTION

The foregoing objects and others which will become apparent from thefollowing description are accomplished in accordance with thisinvention, generally speaking, by providing crosslinking agents whichmay be combined with hydroxyl containing diorganopolysiloxanes to formorganopolysiloxane elastomers comprising a mixture of (1) a productobtained from the reaction of (a) silicic acid esters and (b) an organictin compound of the formula

    R.sub.2 Sn(OCOR.sup.1).sub.2                               (I),

wherein R represents a butyl or an octyl radical and R¹ represents amonovalent hydrocarbon radical having from 1 to 15 carbon atoms, inwhich no more than one of the valences of the carbon atom bonded to thecarboxyl group is saturated by a carbon atoms other than that of thecarboxyl group, and (2) an organic tin compound of the following formula

    R.sub.2 Sn(OCOR.sup.2).sub.2                               (II),

wherein R is the same as above and R₂ represents a monovalent aliphatichydrocarbon radical having from 3 to 15 carbon atoms, in which at leasttwo of the valences of the carbon atom bonded to the carboxyl group aresaturated by at least two carbon atoms other than that of the carboxylgroup.

DETAILED DESCRIPTION OF THE INVENTION

The crosslinking agents of this invention are preferably prepared byreacting any monomer, dimer of polymer of silicic acid esters with anorganic tin compound of formula (I). Examples of preferred silicic acidesters are silicic acid ethyl esters or silicic acid 2-methoxyethylesters. Other examples of silicic acid esters are tetraethyl silicate,hexaethoxydisiloxane and ethoxypolysiloxane having an SiO₂ content ofabout 40 percent by weight, also known as "ethylsilicate 40", as well astetra(2-methoxyethyl)-silicate.

In preparing the reaction products of silicic acid esters and an organictin compound represented by formula (I), it is possible to use a singlesilicic acid ester or a mixture of two or more different types ofsilicic acid esters, for example a mixture consisting of tetraethylorthosilicate and hexaethoxydisiloxane.

It is preferred that the silicic acid ethyl ester be used in an amountof from 2 to 6 parts by weight for each part by weight of the organictin compound of formula (I). The radicals represented by R in formula(I) may be the same or different. Also in formula (II), the R radicalsmay be the same or different. Moreover, the R radicals in the organictin compound represented by formula (II) may be the same or differentfrom the R radicals in the organic tin compound of formula (I).

The radicals represented by R¹ in the organic tin compound of formula(I) may be the same or different. The R² radicals in the organic tincompound of formula (II) may also be the same or different.

The butyl radicals as well as the octyl radicals may be any isomers suchas the n-butyl, sec-butyl, tert-butyl, n-octyl and the 2-ethylhexylradical.

Examples of preferred organic tin compounds represented by formula (I)are di-n-butyltin diacetate, di-n-octyltin diacetate, di-n-butyltindilaurate and di-n-octyltin dilaurate.

In preparing the reaction products of silicic acid esters and an organictin compound represented by formula (I), it is possible to use a singleorganic tin compound or a mixture of two or more such tin compounds.

The reaction products of the silicic acid esters and an organic tincompound represented by formula (I) are prepared by heating a mixturecontaining a silicic acid ester and an organic tin compound to atemperature of from 50° to 200° C. for from 15 minutes up to 15 hours.

Examples of carboxylic acids from which the --OCOR² groups in theorganic tin compounds represented by formula (II) are derived, arealkane acids which are mono- or di-substituted in the alpha-positionrelative to the carboxyl group. Preferred examples of such alkane acidsare 2-ethyl-hexanoic acid, so-called KOCH acids which consist of amixture of carboxylic acids having from 9 to 15 carbon atoms permolecule in which in 90 percent by weight of the acids the carboxylgroup is bonded to a tertiary carbon atom, and2,2,4,4-tetramethyl-1-pentanoic acid. Another acid which may be used is,for example, cyclohexanemonocarboxylic acid.

Examples of preferred organic tin compounds represented by formula (II)are di-n-butyltin diacylate, where the acylate groups are derived from amixture of carboxylic acids having from 9 to 15 carbon atoms permolecule, in which in 90 percent by weight of the acids the carboxylgroup is bonded to a tertiary carbon atoms; a di-n-octyltin diacylatewhere the acylate groups are derived from a mixture of carboxylic acidshaving from 9 to 15 carbon atoms per molecule, in which in 90 percent byweight of the acids the carboxyl group is bonded to a tertiary carbonatom; di-n-butyltin di-2-ethylhexoate and di-n-octyltindi-2-ethylhexoate.

A single organic tin compound represented by formula (II) or a mixturecontaining two or more of such organic tin compounds may be combinedwith the reaction product obtained from the reaction of a silicic acidester and an organic tin compound represented by formula (I).

The tin compound represented by formula (II) is preferably present in anamount of from 0.1 to 0.2 mole per mole of the tin compound representedby formula (I) which was used in the preparation of the reaction productof a silicic acid ester and an organic tin compound of formula (I).

It is preferred that the organic tin compound of formula (II) becombined with the reaction product of the silicic acid ester and theorganic tin compound represented by formula (I) at least during some ofthe time the reaction product is being stored. However, the organic tincompound represented by formula (II) may be added to the reactionproduct only shortly prior to its use.

In addition to the product obtained from the reaction of a silicic acidester and an organic tin compound of formula (I) and an organic tincompound of formula (II), the mixtures of this invention may alsocontain additional substances such as excess silicic acid esters,hydrophobic silicon dioxide having a surface area of at least 20 m² /g,emollients, soluble dyes and fragrances.

Another embodiment of this invention is a process for preparingorganopolysiloxane elastomers which comprises mixing a hydroxylterminated diorganopolysiloxane composition with a mixture containing(1) a product obtained from the reaction of (a) a silicic acid ester and(b) an organic tin compound of the formula

    R.sub.2 Sn(OCOR.sup.1).sub.2                               (I),

wherein R represents a butyl or an octyl radical and R¹ is the same ordifferent and represents monovalent hydrocarbon radicals having from 1to 15 carbon atoms, in which no more than one of the valences of thecarbon atom bonded to the carboxyl group is saturated by a carbon atomother than that of the carboxyl group, and (2) an organic tin compoundof the formula

    R.sub.2 Sn(OCOR.sup.2).sub.2                               (II),

wherein R is the same as above, R² is the same or different andrepresents monovalent aliphatic hydrocarbon radicals having from 3 to 15carbon atoms, in which at least two of the valences of the carbon atombonded to the carboxyl group are saturated by at least two carbonradicals other than those of the carboxyl group.

The same hydroxyl terminated diorganopolysiloxanes which have been orcould have been used heretofore in the preparation of organopolysiloxaneelastomers from diorganopolysiloxanes containing Si-bonded hydroxylgroups, a condensation catalyst and a silicon compound having at leastthree condensable groups per molecule, may be used in the compositionsof this invention. In the present invention, the reaction productconsisting of a silicic acid ethyl ester and an organic tin compound offormula (I) is to be considered as a combination of a condensationcatalyst with a silicon compound containing at least three condensablegroups per molecule. These organopolysiloxanes may be represented by thefollowing general formula:

    HOR.sub.2.sup.3 SiO(SiR.sub.2.sup.3 O).sub.x SiR.sub.2.sup.3 OH

in which R³ represents the same or different, monovalent hydrocarbonradicals or substituted monovalent hydrocarbon radicals and/or polymerichydrocarbon radicals and x represents an integer having a value of atleast 1.

Although this is generally not indicated in the above formula, siloxaneunits other than diorganosiloxane units (SiR₂ ³ O) may be present withinor along the siloxane chain. Examples of such other siloxane units whichare generally present more or less as impurities, are those having theformulas R³ SiO_(3/2), R₃ ³ SiO_(1/2) and SiO_(4/2), where R³ is thesame as above. If such other siloxane units are present, it is preferredthat they be present in an amount less than 1 mole percent.

Examples of hydrocarbon radicals represented by R³ are alkyl radicalssuch as methyl, ethyl, propyl, butyl, hexyl and octyl radicals; alkenylradicals such as vinyl, allyl, ethylallyl and the butadienyl radical; aswell as aryl radicals such as the phenyl radical.

Examples of substituted hydrocarbon radicals represented by R³ arehologenated hydrocarbon radicals such as the 3,3,3-trifluoropropylradical, chlorophenyl and bromotolyl radicals; as well as the cyanoalkylradicals, such as the beta-cyanoethyl radical.

Also, R³ represents substituted and unsubstituted polymeric hydrocarbonradicals and/or copolymers which are bonded to silicon via carbon. Thesepolymeric radicals or copolymers are obtained from the polymerization ofat least one polymerizable monomer containing a carbon-carbon doublebond, such as ethylene, styrene, vinylacetate, n-butylacrylate,n-butylmethacrylate or acrylonitrile. Siloxanes containing polymerichydrocarbon radicals are referred to as modified organopolysiloxanes.

Because they are readily available, it is preferred that at least 80percent of the number of R³ radicals be methyl radicals. Other radicalsrepresented by R³ are preferably vinyl and/or phenyl radicals.

The viscosity of the diorganopolysiloxanes used in the process of thisinvention is generally from 100 to 10⁶ mPa.s at 25° C.

Mixtures of various diorganopolysiloxanes may likewise be employed.

In preparing organopolysiloxane elastomers, a mixture consisting of (1)the product obtained from the reaction of (a) a silicic acid ester and(b) an organic tin compound represented by formula (I) and (2) anorganic tin compound represented by formula (II), is mixed with ahydroxy-terminated diorganopolysiloxane in an amount of from 2 to 10percent by weight, based on the weight of the diorganopolysiloxane to becrosslinked.

In addition to the hydroxyl-terminated diorganopolysiloxane and thecrosslinking agents of this invention, the compositions may also includeother substances that have been or could have been employed heretoforein organopolysiloxane compositions which may be crosslinked to formelastomers. Examples of such other substances are reinforcing andnon-reinforcing fillers, such as pyrogenically produced silicon dioxide,diatomaceous earth, quartz meal, gypsum, precipitated calcium sulfate,aluminum silicate as well as polyvinyl chloride powder, pigments, water,soluble dyes, fragrances, corrosion-inhibitors, emollients such astrimethylsiloxy endblocked dimethylpolysiloxanes which are liquid atroom temperature, polyglycols which may be etherified and/or esterified,and agents which serve to improve the adhesion of the organopolysiloxaneelastomers to the bases on which they are applied, such asepoxyalkylsilanes, as well as solvents.

In the process of this invention, the hydroxyl-terminateddiorganopolysiloxane, the crosslinking agent and any other substanceswhich are generally present in organopolysiloxanes which are cured toelastomers are preferably mixed at temperatures of from about -5° up toabout 40° C., and under atmospheric pressure. However, higher or lowertemperatures and pressures may also be used.

Compositions of this invention are particularly useful in thepreparation of impressions, especially dental impressions, as well asfor the preparation of molded objects and coatings.

In the following examples, all parts and percentages are by weight,unless otherwise specified.

EXAMPLE 1

(a) A mixture containing 3 parts of an ethoxypolysiloxane having an SiO₂content of about 40 percent and 1 part di-n-butyltin dilaurate (formulaI) was heated for 12 hours at 120° C. The product obtained from thereaction of the silicic acid ethyl ester and the dibutyltin dilauratewas cooled and mixed with 5 percent by weight of di-n-butyltin diacylate(formula II) in which the acylate groups were derived from a mixture ofcarboxylic acids containing from 9 to 15 carbon atoms per molecule, inwhich in 90 percent of the acids, the carboxyl groups are bonded to atertiary carbon atom ("dibutyltin diversatate").

Immediately after its preparation, and again after storing for sixmonths at room temperature, the mixture obtained in accordance with thisinvention was incorporated in an organopolysiloxane-filler mixtureconsisting of:

510 g of an Si-bonded hydroxyl terminated dimethylpolysiloxane having aviscosity of 12,000 mPa.s at 23° C.;

510 g of an Si-bonded hydroxyl terminated dimethylpolysiloxane having aviscosity of 1,000 mPa.s at 23° C.;

162 g of a trimethylsiloxy terminated dimethylpolysiloxane having aviscosity of 100 mPa.s at 23° C.; and

690 g quartz meal

at the rate of 3 percent by weight based on the weight of theorganopolysiloxane and filler mixture. The organopolysiloxane-fillermixture had been mixed with 0.3 percent by weight of water, based on theweight of the mixture, and stored for 3 days at room temperature priorto the addition of the silicic acid ethyl ester-organic tin mixtureprepared above. The crosslinking behavior of this mixture was observedat 23° C. and at 50 percent relative humidity. The results are shown inTable 1.

EXAMPLE 2

The procedure described in Example 1 is repeated, except that 5 percentby weight of di-n-butyltin di-2-ethylhexoate was substituted for thedi-n-butyltin diacylate.

EXAMPLE 3

The procedure described in Example 1 was repeated, except that 5 percentby weight of di-n-octyltin di-2-ethylhexoate was substituted for thedi-n-butyltin diacylate.

Comparison Example (a)

The procedure described in Example 1 was repeated, except that no tincompound was added to the reaction product of ethoxypolysiloxane anddi-n-butyltin dilaurate. The reaction product was used at the rate of 5percent by weight based on the total weight of the organopolysiloxaneand the filler.

Comparison Example (b)

The procedure described in Example 1 was repeated, except that 5 percentby weight of di-n-butyltin diacetate, (formula I) was added to thereaction product of ethoxypolysiloxane and di-n-butyltin dilaurate.

EXAMPLE 4

A mixture consisting of 3 parts ethoxypolysiloxane having an SiO₂content of about 40 percent and one part of di-n-butyltin diacetate(formula I) was heated under reflux for 6 hours at a bath temperature of105° C. The product obtained from the reaction of a silicic acid ethylester and the organic tin compound of formula (I) was cooled and thenmixed with 5 percent by weight of the di-n-butyltin diacylate of formula(II) ("di-butyltindiversatate") described in Example 1. The resultantmixture was then added immediately after its preparation and again afterstoring for 6 months at room temperature to an organopolysiloxane-fillercomposition consisting of:

230 g of a dimethylpolysiloxane containing Si-bonded terminal hydroxylgroups and having a viscosity of 20,000 mPa.s at 23° C.;

230 g of a dimethylpolysiloxane containing Si-bonded terminal hydroxylgroups and having a viscosity of 350,000 mPa.s at 23° C.; and

1,420 g precipitated calcium sulfate

at the rate of 0.75 percent based on the total weight of theorganopolysiloxane and filler. The organopolysiloxane-filler mixture hadbeen mixed with 0.3 percent by weight of water, based on the weight ofthe mixture, and stored for 3 days at room temperature prior to theaddition of the silicic acid ethyl ester-organic tin mixture preparedabove. The crosslinking behavior of the mixture was then observed at 23°C. and at 50 percent relative humidity. The results are shown in Table2.

EXAMPLE 5

The procedure described in Example 4 was repeated, except that 5 percentby weight of di-n-butyltin di-2-ethylhexoate (formula II), wassubstituted for the di-n-butyltin diacylate.

EXAMPLE 6

The procedure described in Example 4 was repeated, except that 5 percentby weight of di-n-octyltin di-2-ethylhexoate was substituted fordi-n-butyltin diacylate.

Comparison Example (a')

The procedure described in Example 4 was repeated, except that the onlytin compound employed was the reaction product of ethoxypolysiloxane anddi-n-butyltin diacetate and the reaction product was employed in anamount of 2 percent by weight based on the total weight of theorganopolysiloxane and the filler.

EXAMPLE 7

A mixture containing 3 parts of tetraethyl orthosilicate and 1 partdi-n-butyl diacetate (formula I) was heated for 8 hours under reflux ata bath temperature of 105° C. The product obtained from the reaction ofthe silicic acid ethyl ester and the organic tin compound of formula (I)was cooled and mixed with 5 percent by weight based on the weight of thereaction product of the di-n-butyltin diacylate of formula (II)("dibutyltindiversatate") described in Example 1. The resultant mixturewas then incorporated after its preparation and again after being storedfor 6 months at room temperature in an organopolysiloxane-fillercomposition consisting of:

650 g of a dimethylpolysiloxane containing terminal Si-bonded hydroxylgroups and having a viscosity of 1,000 mPa.s at 23° C., and

350 g diatomaceous earth,

in an amount of 3.2 percent by weight based on the total weight of theorganopolysiloxane and filler mixture. The organopolysiloxane-fillermixture was heated to 135° C. for 8 hours to remove the volatilecomponents, cooled, and then 0.2 percent by weight of water was addedand stored for 3 days at room temperature prior to the addition of themixture containing the silicic acid ethyl ester-organic tin compoundprepared above. The crosslinking behavior of the mixture was thenobserved at 23° C. and at 50 percent relative humidity. The results areshown in Table 3.

EXAMPLE 8

The procedure described in Example 7 was repeated, except that 5 percentdi-n-octyltin diacylate (formula II) in which the acylate groups werederived from a mixture of carboxylic acids containing from 9 to 15carbon atoms per molecule, in which in 90 percent of the acids, thecarboxyl group was bonded to a tertiary carbon atom (a"dioctyltindiversatate") was substituted for the di-n-butyltin diacylatedescribed in Example 1.

EXAMPLE 9

The procedure of Example 7 was repeated, except that 5 percent by weightof di-n-butyltin di-2-ethylhexoate (formula II) was substituted for thedi-n-butyltin diacylate described in Example 1.

EXAMPLE 10

The procedure described in Example 7 was repeated, except that 5 percentby weight of di-n-octyltin di-2-ethylhexoate (formula II) wassubstituted for the di-n-butyltin diacylate described in Example 1.

Comparison Example (a")

The procedure described in Example 7 was repeated, except that no tincompound was used except for the product obtained from the reaction oftetraethyl orthosilicate and di-n-butyltin dilaurate and the reactionproduct was used in an amount of 4 percent by weight based on the totalweight of the organopolysiloxane and the filler.

In the following tables the term "processing time" is the period of timewhich elapses between the time that mixing of components to becrosslinked in the formation of elastomers is initiated, and the timethat crosslinking of the components is first observed.

The figures shown in parentheses in the tables were recorded after themixture containing the reaction product from silicic acid ethyl esterand a tin compound of formula (I) and a tin compound of formula (I orII), had been stored for 6 months. Essentially, the same values wereobtained after the mixtures containing the reaction product of silicicacid ethyl ester and a tin compound of formula (I) and a tin compound offormula (II) had been stored for 4 weeks at 60° C., in accordance withExamples 1 to 10.

                  TABLE 1                                                         ______________________________________                                                       Shore-A-Hardness after                                                Processing                                                                              10     15      30   24                                              Time      min.   min.    min. hours                                    ______________________________________                                        Example                                                                       1        3 min. 20 sec.                                                                            23     30    32   41                                              (3 min. 15 sec.)                                                                          (23)   (29)  (32) (40)                                   2        3 min. 15 sec.                                                                            23     30    31   41                                              (3 min. 20 sec.)                                                                          (22)   (29)  (32) (41)                                   3        3 min. 20 sec.                                                                            22     29    31   41                                              (3 min. 25 sec.)                                                                          (21)   (28)  (31) (41)                                   Comparison                                                                    Examples                                                                      (a)      3 min. 50 sec.                                                                            0      1     5    30                                     (b)      3 min. 50 sec.                                                                            18     28    30   41                                              (5 min. 5 sec.)                                                                           (0)    (6)   (19) (40)                                   ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                       Shore-A-Hardness after                                                Processing                                                                              10     13     15   30   24                                          Time      min.   min.   min. min. hours                                ______________________________________                                        Example                                                                       4        3 min. 30 sec.                                                                            28     36   41   44   58                                          (3 min. 25 sec.)                                                                          (29)   (37) (41) (42) (58)                               5        3 min. 25 sec.                                                                            29     37   41   46   58                                          (3 min. 20 sec.)                                                                          (29)   (39) (43) (47) (58)                               6        3 min. 30 sec.                                                                            25     36   39   48   58                                          (3 min. 30 sec.)                                                                          (24)   (37) (42) (49) (58)                               Comparison                                                                    Example                                                                       (a')     4 min. 5 sec.                                                                             4      17   20   39   59                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                       Shore-A-Hardness after                                                Processing                                                                              10     13     15   30   24                                          Time      min.   min.   min. min. hours                                ______________________________________                                        Example                                                                       7        4 min. 10 sec.                                                                            41     51   53   56   65                                          (4 min. 10 sec.)                                                                          (41)   (51) (53) (56) (65)                               8        4 min. 5 sec.                                                                             36     49   52   56   65                                          (4 min. 5 sec.)                                                                           (37)   (48) (51) (56) (65)                               9        4 min. 5 sec.                                                                             38     49   53   58   66                                          (4 min. 10 sec.)                                                                          (37)   (48) (53) (58) (66)                               10       4 min. 15 sec.                                                                            35     47   51   56   66                                          (4 min. 15 sec.)                                                                          (35)   (48) (51) (57) (66)                               Comparison                                                                    Example                                                                       (a")     5 min.      12     27   30   47   63                                 ______________________________________                                    

The above tables show that the cross-linking behavior of thecompositions of this invention is far less influenced by storage thanthat of other compositions and that the compositions containing themixtures of organic tin compounds crosslink more rapidly.

EXAMPLE 11

A mixture containing 3 parts of tetraethyl orthosilicate and 1 part ofdi-n-octyltin diacetate (formula I) was heated for 3 hours under refluxand at a bath temperature of 120° C. After the bath temperature hadcooled to 50° C., the volatile components were distilled off at 50° C.and at 22 mBar (absolute). The product obtained from the reaction of thesilicic acid ethyl ester and the organic tin compound of formula (I) wasthen mixed with 5 percent by weight of di-n-octyltin di-2-ethylhexoate(formula II). The resultant mixture was then mixed with theorganopolysiloxane-filler mixture described in Example 4 in an amount of3.2 percent by weight, based on the total weight of theorganopolysiloxane and the filler after the organopolysiloxane-fillermixture had been kneaded with 0.3 percent by weight of water and storedfor 3 days at room temperature. The crosslinking behavior of the mixturewas observed at 23° C. and at 50 percent relative humidity. Thefollowing results were obtained.

    ______________________________________                                                     Shore-A-Hardness after                                                          10     13        15   24                                       Processing Time                                                                              min.   min.      min. hours                                    ______________________________________                                        3 min. 45 sec. 35     47        50   65                                       ______________________________________                                    

EXAMPLE 12

A mixture containing 3 parts of tetra-(2-methoxy)-silicate and 1 partdi-n-butyltin dilaurate (formula I) was heated for 3 hours at a bathtemperature of 140° C. After it had cooled, the product obtained fromthe reaction of the silicic acid ester and the organic tin compound offormula (I) was mixed with 5 percent by weight of di-n-butyltindi-2-ethylhexoate (formula II). This mixture was then combined with theorganopolysiloxane-filler mixture described in Example 7, at the rate of2.5 percent, based on the total weight of the organopolysiloxane andfiller after the organopolysiloxane-filler mixture had been heated for 8hours at 135° C. to remove the volatile components, cooled, then mixedwith 0.2 percent by weight of water and stored for 3 days at roomtemperature. The crosslinking behavior of the resultant mixture wasobserved at 23° C. and at 50 percent relative humidity. The followingresults were observed.

    ______________________________________                                                     Shore-A-Hardness after                                                          8      10        15   24                                       Processing Time                                                                              min.   min.      min. hours                                    ______________________________________                                        3 min. 45 sec. 24     35        41   55                                       ______________________________________                                    

Comparison Test

The procedure described in Example 12 was repeated, except that no tincompound was used except for that obtained from the reaction of thesilicic acid ester and di-n-butyltin dilaurate. The reaction product wasused in an amount of 4 percent by weight based on the total weight ofthe organopolysiloxane and the filler. The following results wereobserved:

    ______________________________________                                                     Shore-A-Hardness after                                                          8      10        15   24                                       Processing Time                                                                              min.   min.      min. hours                                    ______________________________________                                        3 min. 20 sec. 0      11        23   52                                       ______________________________________                                    

What is claimed is:
 1. An organopolysiloxane composition capable offorming an elastomer comprising a hydroxyl terminateddiorganopolysiloxane and (1) a product obtained from the reaction of (a)silicic acid ester and an (b) an organic tin compound of the formula

    R.sub.2 Sn(OCOR.sup.1).sub.2                               (I),

wherein R is selected from the group consisting of a butyl and an octylradical, R¹ is a monovalent hydrocarbon radical having from 1 to 15carbon atoms, in which no more than one of the valences of the carbonatom bonded to the carboxyl group is saturated by a carbon atom otherthan that of the carboxyl group and (2) an organic tin compound of theformula

    R.sub.2 Sn(OCOR.sup.2).sub.2                               (II),

wherein R is selected from the group consisting of a butyl and an octylradical, R² is a monovalent aliphatic hydrocarbon radical having from 3to 15 carbon atoms, in which at least two of the valences of the carbonatom which is bonded to the carboxyl group are saturated by at least twocarbon atoms other than that of the carboxyl group.
 2. A process forpreparing an organopolysiloxane elastomer which comprises mixing with ahydroxyl terminated diorganopolysiloxane (1) a product obtained from thereaction of (a) silicic acid ester and (b) an organic tin compound ofthe formula

    R.sub.2 Sn(OCOR.sup.1).sub.2                               (I),

wherein R is selected from the group consisting of a butyl and an octylradical, R¹ is a monovalent hydrocarbon radical having from 1 to 15carbon atoms, in which no more than one of the valences of the carbonatom bonded to the carboxyl group is saturated by a carbon atom otherthan that of the carboxyl group and (2) an organic tin compound of theformula

    R.sub.2 Sn(OCOR.sup.2).sub.2                               (II),

wherein R is selected from the group consisting of a butyl and an octylradical, R² is a monovalent aliphatic hydrocarbon radical having from 3to 15 carbon atoms, in which at least two of the valences of the carbonatom which is bonded to the carboxyl group are saturated by at least twocarbon atoms other than that of the carboxyl group and thereafterexposing the resultant composition to atmospheric moisture.
 3. Theprocess of claim 2, wherein the reaction product (1) is mixed with theorganic tin compound of formula (II) prior to being mixed with thehydroxyl terminated diorganopolysiloxane composition.
 4. The elastomerobtained from the process of claim 2.